1. Field of the Invention
The present invention relates to an optical synthetic quartz glass having excellent light transmissivity, optical homogeneity, and optical stability with respect to the ultraviolet ray irradiation, particularly to the excimer laser irradiation, which is an ultraviolet ray laser, a production method thereof, and an optical member for an excimer laser using the synthetic optical glass.
2. Description of the Related Art
With a higher integration degree of the LSI, processing accuracy of the sub-micron order is required in the photolithography for drawing an integrated circuit pattern on a silicon wafer. Therefore, an exposing device for drawing a circuit pattern is also improved. For example, a finer line width in drawing is achieved by a shorter wavelength of the power source of the exposing device or adopting the resolution enhancement techniques using interference of light. That is, the light source has a short wavelength between the g ray (wavelength 436 nm) to the i ray (wavelength 365 nm) of a mercury lamp, and the exposing device of the diffraction system adopts resolution enhancement techniquies utilizing the interference of light, such as a off-axis illumination method or a phase shift mask method. The demand for finer drawing is accelerated these days so that an excimer laser having a shorter wavelength is used in place of the mercury lamp. Examples of the excimer laser include a KrF laser (wavelength 248 nm) and an ArF laser (wavelength 193 nm). Owing to the use of the excimer laser as the light source, a higher quality is required for optical members including a lens used in the exposing device. For example, if the light transmissivity is poor, the focal length of the lens or the other characteristics suffer adverse effects by the heat generation of the lens due to the light absorption, or if the optical homogeneity is poor, problems such as deterioration of the image formation characteristics are involved.
Conventionally, an optical synthetic quartz glass is used as the material of the optical members for an exposing device for drawing a circuit pattern by a photolithography. The synthetic quartz glass can be produced in the direct method where a vapor of a highly pure silicon compound such as silicon tetrachloride (SiCl4) is introduced directly to the oxygen-hydrogen flame and glass fine particles obtained by the flame hydrolysis are deposited on a heat resistant substrate to be a molten glass for obtaining a transparent glass or in the soot method where the glass fine particles are deposited on a heat resistant substrate as a porous material and heated in an electric furnace to be a molten glass. In either case, by the use of a highly pure material, the transmissivity in the ultraviolet ray region is improved as well as the optical homogeneity can be maintained. In the case of an optical material made from the above-mentioned synthetic quartz glass, damage caused by the light did not have to be considered if the i ray of the mercury lamp is used as the light source of the exposing device, however, it should be dealt with if the excimer laser is used instead. The problem is derived from the pulse energy per one shot of the excimer laser, which is a pulse laser, is extremely large compared with a continuous light source such as the i ray (mercury lamp, CW laser, etc.). Damage caused by the excimer laser on a synthetic quartz glass may differ remarkably depending on the production method or the production conditions thereof. The damage refers to the deterioration of the laser transmissivity caused by the absorption in the ultraviolet region by the ultraviolet ray laser irradiation and the rise of the refractive index caused by the permanent compaction (contraction of the glass). The insusceptibility to the damage is called the laser resistance. As mentioned above, the absorption in the ultraviolet region occurs subject to the damage. This is considered to be because of the paramagnetic defect derived from the intrinsic defect in the quartz glass by the light reaction. The existence of the paramagnetic defect derived from the laser has been observed and identified by the ESR spectrum. As examples thereof, the structures such as El center (Si.) and NBOHC (Sixe2x80x94O.) are known. Such a paramagnetic defect, in general, has an; absorption band. For example, Exe2x80x2 center has it at 215 nm. Furthermore, although the species of the defect has not been identified yet, absorption is observed also at 260 nm subject to the excimer laser irradiation. These absorption bands can be comparatively broad and strong. For example, in the case of the use as the light transmissive material for a KrF laser (wavelength 248 nm) or an ArF laser (wavelength 193 nm), deterioration in the laser transmissivity caused thereby poses a serious problem. In addition to the absorption in the ultraviolet region, a permanent compaction occurs. The compaction is derived from the transition to a more stable structure in a part of the quartz glass due to the atom recombination subject to a strong laser energy irradiation. Accordingly, the density in the irradiated portion heightens to raise the refractive index of the quartz glass material, resulting in a major influence on the image formation characteristics. Furthermore, due to the local density rise in the laser-irradiated portion, the stress is generated at the interface between the non-irradiated portion and the irradiated portion with the distortion so as to raise the birefringence index and affect the optical characteristics.
Various methods have been proposed in order to solve the above-mentioned problems. Example thereof include a method of having particular production conditions of a quartz glass, and a method of applying a heat treatment to a produced synthetic quartz glass in a particular atmosphere. As an example of the former method, Japanese Unexamined Patent Publication Nos. 6-199531 and 6-287022 disclose a production method with a hydrogen-excessive condition, paying attention to the gas balance in the synthetic quartz glass production. By having the hydrogen molecules dissolved as mentioned above, the paramagnetic defect caused by the laser irradiation can be compensated by the hydrogen molecules so that the generation of the paramagnetic defect can be restrained and thus the laser transmissivity can be ensured without generating an absorption band in the ultraviolet ray region. In the method of dissolving hydrogen molecules, the laser resistance improves with a larger amount of dissolved hydrogen molecules in producing a synthetic quartz glass. However, since the amount of the hydrogen molecules to be dissolved remarkably varies according to not only the gas amount of a combustion gas and a combustion-supporting gas but the surface temperature or the surface area of the substrate to be deposited during the growth in the direct method, it is difficult to control the factor. Therefore, hydrogen molecules dissolved more than intended may result in a strongly reducing property to generate an oxygen lacking defect or a reduced species of silicon atoms (xe2x95x90Si:), which provides a precursor of the paramagnetic defect and deteriorates the laser resistance. Although the above-mentioned method of dissolving hydrogen molecules is advantageous in terms of the improvement of the laser resistance, with the laser irradiation for a very long time to the synthetic quartz glass having hydrogen molecules dissolved, the ultraviolet ray absorption occurs due to the dissolved hydrogen molecule consumption. Since the absorption is derived from the paramagnetic defect, a production method of a quartz glass for minimizing the paramagnetic defect structure is discussed. Examples of structures to cause the paramagnetic defect include (i) one derived from a glass structure, such as an unstable SiO2 network portion, (ii) an unordered structure generated from the deviation in the stoichiometric ratio, such as Sixe2x80x94Si and Sixe2x80x94Oxe2x80x94Oxe2x80x94Si, (iii) a structure excluding silica, such as SiCl and SiOH, and (iv) an unordered structure derived from a metal impurity. Japanese Unexamined Patent Publication No. 7-61823 discloses a production method of a synthetic quartz glass with little amount of such a structure. The method is to have the growth ratio of a quartz glass ingot to be 2 mm/hour or less. However, since the glass growth rate is too low, it results in a poor productivity and a high production cost. Furthermore, the conventional synthetic quartz glass may generate a paramagnetic defect based on SiCl. In the conventional production method of a synthetic quartz glass, since silicon tetrachloride is used basically as the material, and silica fine particles are generated by the hydrolysis reaction thereof in an oxygen-hydrogen flame to have a molten glass, unreacted SiCl remains. The residual amount of SiCl varies depending upon the oxygen-hydrogen flame conditions, and the temperature in the growths surface, that is, the depositing and melting conditions. In general, it remains about 10 to 150 ppm and it is difficult to have its concentration less than the detection limit. In addition to the residual SiCl, in the conventional production method of a synthetic quartz glass, a hydrogen chloride gas is generated in the production. Since the hydrogen chloride gas is hazardous, it should be eliminated. Besides, since it erodes the device, an erode prevention means needs to be provided, and thus the necessity soared the production cost.
On the other hand, optical members used in the exposing device with a photolithography technique, such as a lens and a prism need to have the laser resistance. In addition, it is also important to have excellent light transmissivity, optical homogeneity without generation of fluorescence, bubbles, or distortion, or inclusion of a foreign matter. Regarding the optical homogeneity, it is required even for a member larger than a 200 mm diameter size not to have a stria, and to have a refractive index difference (xcex94n) of 2xc3x9710xe2x88x926 or less. In general, in a production method of a synthetic quartz glass with a single burner, a material is introduced from one direction onto a rotating target with an oxygen-hydrogen flame blown so as to deposit and melt silica fine particles and thus a temperature distribution is generated on the growth surface. That is, a portion where directly applied with the flame has a relatively high temperature, but on the other hand, a portion on the opposite side has a relatively low temperature. Since the target is rotated with a predetermined rotation frequency, a certain portion has a temperature change as time passes with a cycle of a high temperature and a low temperature successively alternated according to the rotation frequency of the target. Glass is deposited and grows on the rotation axis accordingly. If silica fine particles of a high temperature are blown to a portion having a low temperature and re-melted, the interface is not homogeneous but the density and physical properties differ thereat from the microscopic viewpoint so that the interface is observed as the stria like a layer along the rotation axis direction. In order to restrain the stria generation, the production conditions need to be improved. As a method therefor, (A) to have the temperature distribution in the growth surface homogeneous, (B) to maximize the temperature in the growth surface, and (C) to minimize the amount of attached silica fine particles per one rotation of the target can be presented. In the direct method using one burner, it is difficult to have the temperature distribution in the growth surface homogeneous. Therefore, a method of surrounding the growth surface with a heat resistant container to seal the heat has been proposed. However, the method has a disadvantage in that the flame is disturbed and thus a stable continuous growth cannot be conducted. Furthermore, in the method of maximizing the growth surface, with an excessively high temperature, the viscosity of a quartz glass becomes too low to keep the shape of the growth surface and the shape differs in the distance from the burner to the target to cause an irregular quality. In the worst case, a continuous growth cannot be conducted. Besides, since a noncombustible silicon compound such as silicon tetrachloride is used as the material in the conventional production method of a synthetic quartz glass, the frame temperature of the burner tends to be lower and thus it is difficult to maintain the temperature of the silica fine particle growth surface at a high level. Therefore, a large amount of a combustion gas needs to be introduced to the burner. It results in a higher gas flowing rate of the burner to dent a portion which is applied to the flame of the burner directly to cause the growth surface shape change. As a result, generation of a stria becomes more liable.
In consideration of the problems of the conventional production methods of a synthetic quartz glass, the present inventors have studied elaborately to find out that a synthetic quartz glass having excellent light transmissivity and optical homogeneity and a high laser resistance can be produced with a high productivity by using an organosilazane compound as the material and introducing it into a flame comprising a combustion gas and a combustion-supporting gas to generate silica fine particles, and depositing the silica fine particles onto a rotating heat resistant substrate to be a molten glass, and completed the present invention.
A synthetic quartz glass of the present invention is excellent in terms of the excimer laser resistance, in particular, the ArF laser resistance. The synthetic quartz glass is advantageous as an optical member, such as a lens, a prism, and a beam slitter for an exposing device for a stepper having an excimer laser as the light source.
An object of the present invention is to provide an optical synthetic quartz glass, having excellent light transmissivity and optical homogeneity and a high excimer laser resistance.
Another object of the present invention is to provide an optical synthetic quartz glass having excellent laser transmissivity and optical homogeneity and a high ArF laser resistance.
Yet another object of the present invention is to provide a production method of the synthetic quartz glass.
Still another object of the present invention is to provide an optical quartz glass member having a high excimer laser resistance, using the synthetic quartz glass.